1. Field of the Invention
This invention relates to a hydraulic control device, in particular, to a hydraulic control device for an automatic transmission having a check valve which drains centrifugal hydraulic pressure developed in a chamber of a hydraulic actuator. In detail it relates to a device to control supply of hydraulic pressure to a hydraulic actuator.
2. Description of the Prior Art
Generally, a hydraulic actuator for an automatic transmission engages and releases frictional engaging elements such as clutches, by supply and drain of hydraulic pressure In the case of a device having a hydraulic actuator installed on a rotating element, centrifugal hydraulic pressure works on oil in a hydraulic chamber of the hydraulic actuator, which makes it difficult to release clutches because oil remains in the hydraulic chamber, even the valve is switched to drain oil in the chamber.
For the purpose of draining the remained oil, a check valve having a ball in a circular cone hole is placed in a piston. The hydraulic pressure works on the ball to close the hole when the pressure works on the hydraulic chamber, and the ball detaches from the hole by centrifugal force to drain hydraulic pressure when draining.
However, in case that the capacity of an orifice made at an oil passage which is connected to the hydraulic chamber of the hydraulic actuator is small, it takes quite a time to raise hydraulic pressure, which run through the orifice, so the slip time is made longer, and, as a result, the clutch may burn. Also, when rotation increases, hydraulic pressure raise can not overcome centrifugal force working on the ball, so that initial supply of hydraulic pressure is disabled and the clutch may be under non-engaging condition. On the other hand, in case that capacity of the orifice is large, shift shock may happen because of sharp increase of hydraulic pressure especially when rotation speed is low.
For the purpose of preventing centrifugal hydraulic pressure generated in the hydraulic chamber of the clutch actuator during rotating at a high speed, the following structure has been proposed: as shown in FIG. 5, baffle plates 41,41 are installed near the check valve 25 made in a piston 40. By these baffle plates 41,41, occurrence of swirl is prevented, so that increase of hydraulic pressure in the hydraulic chamber, which works on the ball, is practiced securely and closing of the check valve 25 is done rapidly.
As shown in FIG. 6, also another structure has been proposed: a plate 44 is fixed on the rigid member 43 without movement, the plate 44 is covered by a back side of the piston 45 oil-tightly, and spring 49 is installed between the plate 44 and the piston 45, so that a ballancing chamber 48, opposing a hydraulic chamber 47 formed by the piston 45 and the clutch 46, is formed. By this structure the hydraulic pressure is supplied to the balancing chamber 48 from an oil path connected to the actuator 47. So this pressure which becomes centrifugal hydraulic pressure in the chamber 48 counterbalances the centrifugal hydraulic pressure working on the hydraulic pressure chamber 47.
However, the baffle plate is not enough to get rid of the centrifugal hydraulic pressure even if occurrence of swirls is prevented and the closing of the check valve is done rapidly. Especially when the hydraulic actuator rotates at high speed, slip time of the frictional clutch becomes longer, which may result in burning of the frictional discs.
In case that the ballancing chamber is employed, the check valve can be eliminated, however, the structure becomes complicated, so that problems of maintenance may happen, axial length may increase, and mounting space may not correspond to the recent trend of front-engine-front-drive cars which requires much narrower space for mounting.